To delete a project, you must be the Account owner or Organisational Administrator (for the org that owns the project). For more information on this, please consult the User permissions article.

As an administrator that owns the project, select the project you'd like to delete from your dashboard and then access 'Settings' from your sidebar menu, then in the 'General' section, choose 'Delete' as seen below.

Now select the big red 'Delete' button and that's it.

You can also delete projects in bulk by using the organisation menu, as shown here.

NB Only delete your project if you're sure you won't need any of its contents ever again! You can always delete individual files from a project via the Media page...